Mice transgenic for the human myotonic dystrophy region with expanded CTG repeats display muscular and brain abnormalities

The autosomal dominant mutation causing myotonic dystrophy (DM1) is a CTG r epeat expansion in the 3'-UTR of the DM protein kinase (DMPK) gene. This mu ltisystemic disorder includes myotonia, progressive weakness and wasting of skeletal muscle and extramuscular symptoms such as cataracts, testicular a trophy, endocrine and cognitive dysfunction. The mechanisms underlying its pathogenesis are complex. Recent reports have revealed that DMPK gene haplo insufficiency may account for cardiac conduction defects whereas cataracts may be due to haploinsufficiency of the neighboring gene, the DM-associated homeobox protein (DMAHP or SIX5) gene. Furthermore, mice expressing the CU G expansion in an unrelated mRNA develop myotonia and myopathy, consistent with an RNA gain of function. We demonstrated that transgenic mice carrying the CTG expansion in its human DM1 context (> 45 kb) and producing abnorma l DMPK mRNA with at least 300 CUG repeats, displayed clinical, histological , molecular and electrophysiological abnormalities in skeletal muscle consi stent with those observed in DM1 patients. Like DM1 patients, these transge nic mice show abnormal tau expression in the brain. These results provide f urther evidence for the RNA trans-dominant effect of the CUG expansion, not only in muscle, but also in brain.